THE DIENCEPHALIC FLOOR 263 
nerve, the supraoptic recess (42). The caudal portion of the 
infundibular region presents no change of moment. The two 
sprouts of the pituitary anlage which extend along the ventral 
surface of the median post-chiasmatic groove are still independ- 
ent of each other except for a small area near their point of origin 
where they seem to have undergone fusion across the median 
line. The mammillary region (25) has extended to some degree 
laterally so that now there are added to the original median 
evagination of this region two slightly projecting lateral proc- 
esses into which the cavity of the ventricle extends. In the 
70 mm. Mustelus the inferior lobes are more prominent and the 
pituitary sprouts extending along the ventral surface of the me- 
dian post-chiasmatic groove have now united throughout their 
entire length, thus forming a tongue-like projection of the pitui- 
tary gland. The mammillary region in this stage has attained 
all of the characteristics of the posterior lobe (27), presenting a 
median portion and two lateral processes which project free. 
In the caudal area of the infundibular region the only notable 
change is in the apparent expansion of. the ventral evagination 
in that portion immediately ventral to the dorsal evagination. 
This is an area which is not in contact at any point with the 
pituitary gland. 
Mustelus of 100 and 3800 mm.; Specimens Nos. 726 and 694 
(figs. 29 and 30). In both of these late stages the angle in the 
neural axis at the posterior isthmian sulcus has disappeared. 
The optic chiasm (4) and chiasmatic process are larger, thus 
accentuating the prechiasmatic (38), and the post-chiasmatic 
(33) recesses. The supraoptic recess (42) extends as a small 
canal along the cephalic surface of the chiasm and the proximal 
portion of the optic nerve. In the post-chiasmatic eminence 
(32) the inferior lobes and the median post-chiasmatic groove 
may be definitely recognized. In the groove rests the tongue- 
like extension of the pituitary gland. The posterior lobe (27) 
has increased in size, especially in its lateral processes, each of 
which contains an accessory recess of the ventricle. More 
prominent changes,, however, have appeared in the caudal 
portion of the infundibular region and particularly in the ventral 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 25, NO. 3 
